A parallel optical communications module may be a parallel optical transceiver module, a parallel optical transmitter (Tx) module, or a parallel optical receiver (Rx) module. A parallel optical Tx module includes a plurality of laser driver circuits and a plurality of respective laser diodes. Each laser driver circuit outputs an electrical drive signal to its respective laser diode to cause the respective laser diode to be modulated. When the laser diode is modulated, it outputs optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system of the parallel optical Tx module directs the optical signals produced by each respective laser diode into the end of a respective transmit optical fiber held within an optical connector module that connects to the parallel optical Tx module. The optical fibers are typically part of an optical fiber ribbon cable.
A parallel optical Rx module includes a plurality of receive photodiodes that receive respective incoming optical signals output from the ends of respective receive optical fibers, which are typically also part of an optical fiber ribbon cable. An optics system of the parallel optical Rx module focuses the light that is output from the ends of the receive optical fibers onto the respective receive photodiodes. The respective receive photodiodes convert the respective incoming optical signals into respective electrical analog signals. Electrical detection circuits, such as transimpedance amplifiers (TIAs), receive the respective electrical analog signals produced by the respective receive photodiodes and output corresponding amplified electrical signals, which are processed by other circuitry of the Rx module to recover the data.
A parallel optical transceiver module includes an optical Rx and an optical Tx for simultaneously transmitting and receiving optical data signals over the optical fibers of one or more optical fiber ribbon cables.
The optical fiber ribbon cables used in parallel optical Tx, Rx and transceiver modules typically have ends that are terminated with optical connector modules, which are configured to be plugged into a receptacle of the optical Tx, Rx or transceiver module. Some parallel optical communications systems comprise a plurality of parallel optical Tx, Rx, or transceiver modules that are arranged inside of a system housing. In such arrangements, each parallel optical Tx, Rx or transceiver module is connected to a respective optical connector module that holds the end of a respective optical fiber ribbon cable. The opposite ends of the optical fiber ribbon cables pass out of the housing through an opening formed in the housing. Therefore, this opening must be at least large enough in width and height to accommodate the width and height of a stack of two or more optical fiber ribbon cables.
In such arrangements, the opening in the housing through which the stack of ribbon cables passes constitutes an EMI open aperture that allows EMI to escape from the housing. The Federal Communications Commission (FCC) has set standards that limit the amount of electromagnetic radiation that may emanate from unintended sources. For this reason, a variety of techniques and designs are used to shield EMI open apertures in such housings in order to limit the amount of EMI that passes through the apertures. Various metal shielding designs and resins that contain metallic material have been used to cover areas from which EMI may escape from the housings. So far, such techniques and designs have had only limited success, especially with respect to parallel optical communications modules that transmit and receive data at high data rates (e.g., 10 gigabits per second (Gbps) and higher).
The amount of EMI that passes through an EMI shielding device is proportional to the largest dimension of the largest EMI open aperture of the EMI shielding device. Therefore, EMI shielding devices are typically designed to ensure that there is no open aperture that has a dimension that exceeds the maximum allowable EMI open aperture dimension associated with the frequency of interest. However, as indicated above, the size of the opening in the housing through which the stack of ribbon cables passes must be at least large enough to accommodate the stack of ribbon cables. Therefore, the opening constitutes an EMI open aperture that is much larger than the maximum allowable EMI open aperture dimension of the optical communications system, particularly at high bit rates. Consequently, unacceptable amounts of EMI may escape from the optical communications system through the opening.
One technique that is sometimes used to provide EMI shielding at the opening in the housing involves placing a metal EMI shielding device in the housing surrounding the opening such that the ribbon cables pass through the EMI shielding device. While such shielding devices are relatively effective at preventing EMI from passing through regions in the housing immediately adjacent the opening, they are totally ineffective at preventing EMI from passing through the opening itself, which is filled only with the fibers and air. Of course, the fibers and the air are transparent to EMI.
Accordingly, a need exists for a way to provide an effective EMI containment solution that significantly limits the amount of EMI that is allowed to pass through this opening in the housing.